Continuous carbothermal reactor

ABSTRACT

By supplying gaseous nitrogen throughout a discrete aliquot of a preferably pelletized mixture of aluminum oxide, carbon and, optionally, calcium oxide during the carbothermal reduction thereof to aluminum nitride and continuously removing gaseous reaction products therefrom, a high quality aluminum nitride is produced. One means of supplying gaseous nitrogen to the mixture of solid reactants is a perforated tray having a hollowed-out bottom. Gaseous nitrogen supplied to the hollowed-out portion flows through the perforations and throughout solid reactants contained in the tray. The carbon may be alternatively supplied, in whole or in part, as a gaseous reactant.

BACKGROUND

The present invention concerns an improved method for preparing aluminumnitride powders. The present invention also concerns an apparatussuitable for use in conjunction with the improved method.

Aluminum nitride exhibits certain physical properties which make itparticularly suitable for use in a variety of applications. Someapplications, e.g., packaging components for electronic circuitry,require substantially full theoretical density and high thermalconductivity. High quality aluminum nitride powder, when densified bysintering, hot-pressing or other suitable means, generally satisfiesthese requirements. A number of factors contribute to powder quality.Powder particle size and surface area primarily affect density of theresultant ceramic article. Powder purity plays a major role indetermining purity of the resultant ceramic and thereby the magnitude ofcertain physical properties such as thermal conductivity.

High quality aluminum nitride powder typically has a low oxygen content(less than about 2%), a low carbon content (less than about 0.2%), andlow trace metals content (less than a few hundred parts per million).Lower quality aluminum nitride powders, e.g., those with greater oxygen,carbon or trace metals contents, are generally regarded as unsuitablefor use in certain electronics applications such as electronicpackaging. Sinterable aluminum nitride powders typically have a particlesize of from 1.0 to 0.2 micrometers inclusive. The surface area of thepowders, being inversely proportional to the particle size, ranges fromabout 2 to 10 m² /g inclusive.

Production of aluminum nitride powder typically follows one of two knownmethods. One method, known as direct nitridation, involves nitriding ofmetallic aluminum nitride powder in a nitrogen or ammonia atmosphere athigh temperature and pulverizing the resultant nitride. The secondmethod, known as carbothermal reduction, reacts aluminum oxide, carbonand nitrogen at a high temperature. The present invention focuses uponthe latter method.

An examination of the carbothermal reduction reaction thermochemistryshows that it has a highly endothermic nature under all conditions. Assuch, heat must be supplied in an effective and efficient manner if thereaction is to proceed at an acceptable velocity. Adverse effects of animproper supply of heat include an incomplete reaction of startingmaterials, coarsening or grain growth of the aluminum oxide startingmaterial or the aluminum nitride product or both, and undesirable sidereactions to form unwanted byproducts such as aluminum oxynitride.

Complete conversion of the reactants requires both an effectiveintroduction of reactant gases, e.g., nitrogen, into the reacting massand an efficient removal of product gases such as carbon monoxidetherefrom. If reactant gas introduction and product gas removal are notdone properly, the resultant aluminum nitride product can contain highlevels of oxygen. Excess oxygen indicates that the reaction has achievedan equilibrium position between starting materials and products whichlies short of complete conversion to the desired aluminum nitride.

Kuramoto et al. (U.S. Pat. No. 4,618,592) teach the importance ofchoosing and maintaining high purity in the reactant solids, e.g.,aluminum oxide and carbon. They also teach the importance of preparingan intimate mixture of the reactant solids. Their Example 1 discloses asmall (30 to 200 gram) scale reaction in an electric furnace operatingat about 1600° C. while feeding nitrogen gas into the furnace at a rateof 3 liters per minute. Following a reaction time of 6 hours, themixture is removed and oxidized in air to remove unreacted carbon.

Reaction conditions suitable for use in conjunction with a laboratoryscale reactor may not provide acceptable results in a larger scaleapparatus. A small reacting mass allows for relatively efficient gas andthermal transport which, in turn, lead to preparation of high qualitypowders even under far less than ideal conditions. As the size of thereaction vessel increases to accommodate larger reacting masses,degradation of gas and thermal transport efficiency usually follows. Asthe depth of a bed of reactant solids increases, difficulties inproviding contact between reactant solids and reactant gases and removalof product gases change from minor irritants to major problems. As thebed of reactant solids increases in size, heating of the bed to drivethe endothermic reaction toward completion becomes increasinglynon-uniform and varies with the distance of a portion of the bed fromthe source of heat. In other words, the reaction proceeds from theoutside of the reactant bed or charge toward its center in response toan external source of heat. The foregoing gas and thermal transportproblems give rise to less than ideal reaction conditions in localvolumes within a reacting mass and consequent variability in aluminumnitride conversion and quality.

Design and operation of a reactor or process to provide near idealreaction conditions in the reacting mass, while necessary, are notsufficient for a successful scale-up of aluminum nitride synthesis toindustrial scale. Other factors, including raw materials, labor andutilities, must be managed efficiently in order to manufacture acompetitive product.

Although operation of a continuous reactor or process may provide a costeffective use of utilities and labor, it also necessarily implies motionor moving parts. The design and operation of a reactor with hot movingparts is limited by the availability and performance of suitablematerials. Addressing this limitation, while necessary, may give rise toother problems such as unacceptable loss of reaction control and productquality.

A number of references describe reactors and processes for preparingaluminum nitride Some suggest the potential for industrial scalereaction of aluminum oxide with carbon and nitrogen. Others addressreaction scale without reference to the product quality. Although a fewreferences bring up the need for complete conversion of reactants to aproduct containing low oxygen, none address industrial scale facilitiesand processes for preparing high quality aluminum nitride having bothlow oxygen and fine particle size. Indeed, references which stress scaleand product oxygen content necessarily preclude attainment of a fineparticle size material.

Kuramoto et al., supra, disclose a process which prepares high qualitypowder. The process is not, however, suitable for practice on anindustrial scale. Static beds of powdered solid reactants areimpractical for large scale operations due to problems with productquality and uniformity and uneconomical reaction kinetics. Reactiontimes of six hours or more are clearly excessive.

Serpek (U.S. Pat. No. 888,044) discloses a method of producing aluminumnitride which consists of heating a mixture of alumina, carbon and ametal capable of forming an alloy with aluminum in a nitrogenousatmosphere to red heat. The resultant product quality is less thandesirable because of contamination due to retained metal.

Serpek (U.S. Pat. No. 1,030,929) teaches the use of an electric furnacein which raw material powder mixtures are introduced into a rotaryreaction chamber heated by resistance elements. Conversion of themixtures to aluminum nitride is assisted by a counter flow of gaseousnitrogen. The rotary action of the chamber provides necessary agitationof the powder mixtures. This facilitates both gas and thermal transport.However, it also leads to unacceptable mixing of unreacted, partiallyreacted and fully reacted materials. If the reactor is operated at feedrates and residence times sufficient to fully convert all of theunreacted materials in such a mixture, the resultant material is stillnot uniform. The lack of uniformity translates to an unacceptableproduct.

Serpek (U.S. Pat. No. 1,078,313) teaches incorporation of hydrogen intothe nitrogenous reaction atmosphere to induce somewhat faster initialreaction kinetics. However, the best product shown in the examplescontains only 8.6 percent nitrogen, an indication of a conversion ofapproximately 30 percent.

Shoeld (U.S. Pat. No. 1,274,797) teaches a process for producingaluminum nitride which utilizes a vertically situated reaction zonethrough which briquets of aluminum oxide and carbon and a binder arepassed while a nitrogen containing gas is uniformly distributed within.The reacting mass is heated by means of electrodes which cause currentto pass through the briquets, heating each directly and uniformly. Theconfiguration and operation of this process places severe demands uponthe composition and physical properties of the feed briquets and on thepartially and completely reacted briquets as well. In order for thebriquets to pass electricity, the composition must be precisely tailoredto provide the correct resistance. Unfortunately, the resistance clearlychanges in an unpredictable fashion as the material is reacted. Thisunpredictability leads to inefficient heating of the reacting masswhich, in turn, leads to variable reaction kinetics and nonuniformproduct quality. In addition, a vertical deep bed of briquets placessevere constraints on briquet strength. The briquets must have both highunreacted strength and sufficient strength during conversion to avoiddisintegration and consequent blinding of the column to flow of gaseousnitrogen. High strength is usually provided by incorporation of largeamounts of binder or by the preparation of a denser material. Largeamounts of binder compromise the purity of the product or change thecourse of the reaction whereas denser feed briquets inhibit thenecessary gas transport within the briquet resulting in longer reactiontimes or lower product quality or both.

Perieres et al. (U.S. Pat. No. 2,962,359) teach the importance ofmaintaining effective control of the atmosphere flow and composition inall portions of the reacting mass including the volume within individualporous briquets. The briquets consist of aluminum oxide and aluminumoxide in admixture with coke. Perieres et al. also teach the existenceof volatile solid byproducts which can clog the reactor and otherwisealter the reaction's critical stoichiometry.

Clair (U.S. Pat. No. 3,032,398) discloses a process for continuouslyproducing aluminum nitride. The process comprises forming a particulatefeed material composed of aluminum oxide, carbon and a calcium aluminatebinder; continuously passing the particulate material downward into anexternally heated elongated reaction zone; passing a countercurrent flowof nitrogen through the descending particulate material; and removingand recovering the aluminum nitride below the reaction zone. The exhaustgases are conducted through an expansion zone to condense any calciumcontained in the gases. The volatilized calcium compounds, if notremoved, would otherwise clog the reactor. Some calcium remains in theproduct and represents an undesirable impurity. The binder also causesexcessive sintering of particulate material thereby preventing recoveryof a fine particle size product. Because nitrogen is consumed in thereaction and carbon monoxide is released, the elongated reaction zonewith its axial flow of gas necessarily contains a non-uniform reactionatmosphere. In addition, the mechanical nature of the particulate flowwithin a stationary tube results in a nonuniform distribution ofparticle velocities leading to an uncertain residence time. Furthermore,countercurrent gas typically flows via channels within a deep orelongated bed. Such flow patterns contributes to production of anonuniform product.

Paris et al. (U.S. Pat. No. 3,092,455) disclose a process for producingaluminum nitride wherein aluminum oxide grains are contacted with areactant gas containing a hydrocarbon as a source of carbon. The processmay be used in conjunction with a fixed bed reactor, a moving bedreactor, or a fluidized bed reactor. The introduction of a hydrocarboninto a fixed or moving bed of aluminum oxide grains, in either aco-current or a countercurrent flow, results in a nonuniformdistribution of carbon, a critical reactant. The resultant product isexpected to be similarly nonuniform. The fluidized bed typicallyprovides for rapid and uniform mixing of the solid and gaseousreactants. However, continuous operation of a fluidized bed mandatescontinuous removal of product. The product so removed contains a finite,but undesirable, amount of unreacted and partially reacted solids.

SUMMARY OF THE INVENTION

One aspect of the present invention is a method for continuouslyproducing aluminum nitride via carbothermal reduction of aluminum oxideThe method comprises:

(a) providing at least one discrete aliquot of solid reactant materials,each aliquot being disposed within a separate container, each containerhaving defined therein a means for receiving gaseous reactants anddistributing said gaseous reactants in a generally uniform mannerthroughout said aliquot, said solid reactants comprising aluminum oxideand, optionally, carbon, said gaseous reactants comprising nitrogen and,optionally, a source of carbon;

(b) passing the container(s) at least once through a heated reactionzone at a rate and for a period of time sufficient to heat said aliquotto a temperature sufficient to initiate a reaction between the gaseousreactants and solid reactant materials to produce aluminum nitride;

(c) supplying the gaseous reactants to said container at a ratesufficient to convert the solid reactant materials to aluminum nitride;

(d) removing gaseous reaction products from the solid reactant materialsat a rate sufficient to form aluminum nitride with a controlled particlesize and substantially preclude formation of aluminum oxynitride oraluminum oxycarbide.

The solid reactants are suitably in a form which maximizes contact withgaseous reactants and removal of reactant gases. The actual form isimmaterial and may be selected from the group consisting of powder,flakes, pellets, agglomerates and the like. The solid reactants aredesirably in the form of pellets.

A second aspect of the present invention is an apparatus comprising:

(a) means for containing solid reactant materials, said means includinga means for receiving gaseous materials and distributing said gaseousmaterials throughout any solid reactant materials contained therein;

(b) means for conveying said container means at least once through areaction zone;

(c) means for supplying gaseous materials to the means for receivinggaseous materials said container means while the container means areconveyed through the reaction zone; and

(d) means for removing gaseous reaction products from the reaction zone.

The method and apparatus of the present invention suitably produce highquality aluminum nitride powder on a commercial scale. The resultantaluminum nitride beneficially has an oxygen content equal to or lessthan about 1.5% by weight, a particle size of equal to or less thanabout 1.5 microns and a specific surface area of between about 2 m² /gmsand about 5 m² /gms. Sintered aluminum nitride formed from aluminumnitride powder prepared by the method and apparatus of the presentinvention can be of near theoretical density with a thermal conductivityin excess of about 140 W/m° K.

The above features and other details of the invention, either as stepsof the invention or as combinations of parts of the invention, are moreparticularly described with reference to the accompanying drawings andpointed out in the claims.

Particular embodiments of the invention are shown by way of illustrationonly and not as a limitation of the invention. Principal features of theinvention may be employed in various embodiments without departing fromthe scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional side view of a schematic illustration of oneembodiment of the apparatus of the present invention

FIG. 2 is a cross-sectional plan view taken along line 2--2 of FIG. 1.

FIG. 3 is a cross-sectional side view of schematic illustration of ameans for containing solid, finely-divided materials. The means isadapted to receive gaseous reactants by countercurrent flow thereof.

FIG. 4 is a cross-sectional side view of a schematic illustration of apreferred means for containing solid, finely-divided materials. Themeans is adapted to receive gaseous reactants via a porous bottom.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 schematically depict an apparatus suitable for purposes ofthe present invention and designated by the reference numeral 10. Theapparatus 10 comprises a reaction chamber 20, a plurality of upperreaction chamber conduits 28, a plurality of lower reaction chamberconduits 29, a first tunnel chamber 30, a first tunnel chamber conduit35, a second tunnel chamber 40, a second tunnel chamber conduit 45, afirst air lock 50, a second air lock 55, a container support means(rail, track, path or channel) 60, a plurality of container means, boatsor trays 70, a plurality of heating elements 80, a means (not shown) ofmoving said container means 70 along track 60, a source of reactantgases (not shown) and a means for receiving gaseous reaction products(not shown).

The first tunnel chamber 30 has a first end 31 located proximate to, andin operative communication with, the first air lock 50 by way of innerair lock door 52 and a second end 32 located remote from the first end31 and proximate to, and in fluid communication with, first end 21 ofreaction chamber 20. The first tunnel chamber conduit 35 is connectedto, and in fluid communication with, the first tunnel chamber 30.

The second tunnel chamber 40 has a first end 41 located proximate to,and in fluid communication with, second end 22 of reaction chamber 20,and a second end 42 located remote from the first end 41 and proximateto, and in operative communication with, the second air lock 55 by wayof inner air lock door 56. The second tunnel chamber conduit 45 isconnected to, and in fluid communication with, the second tunnel chamber40.

The reaction chamber 20 has disposed therein the heating elements 80.The heating elements 80 are arrayed in a manner sufficient to impartheat to contents of container means 70 as said means traverse reactionchamber 20. As shown in FIG. 1, heating elements 80 are arrayed abovecontainer means 70. That arrangement may be varied as desired to placethe heating elements 80 proximate to the sides of, below, or all aroundsaid container means 70 while they are disposed within reaction zone 20.

The placement of heating elements 80 also fixes the first end 21 and thesecond end 22 of reaction zone 20. As shown in FIG. 1, the first end 21is proximate to the heating element 80 located closest to the first airlock 50 and the second end 22 is proximate to the heating element 80located closest to the second air lock 55.

If desired, the heat supplied by heating elements 80 may be supplementedby preheating reactant gases by a means (not shown) before introducingthem into reaction chamber 20. Sweep gases, if used, could also bepreheated. Preheating apparatus and the operation thereof are known.Suitable preheat temperatures are readily determined without undueexperimentation.

The upper reaction chamber conduits 28 are connected to, and in fluidcommunication with, reaction zone 20. The lower reaction chamberconduits 29 are connected to the rail, track, path or channel 60 in sucha manner that the conduits 29 are in fluid communication with a lowerportion of reaction zone 20. At least one of the conduits 29 isbeneficially in fluid communication with a gaseous reactant receivingchamber 71 of each container means 70 while said container means 70 isdisposed within reaction zone 20. As shown in FIG. 1, and moreparticularly in FIG. 4, the chamber 71 is suitably a hollowed out lowerportion of container means 70. The edges of the hollowed out portionbeneficially form a frictional seal with channel 60 sufficient tominimize loss of gaseous reactants (not shown) introduced into chamber71 via one or more of conduits 29.

The first air lock 50 has an inner air lock door 52 located proximateto, and in operative communication with, the first end 31 of the firsttunnel chamber 30 and an outer air lock door 51 located remote from theinner air lock door 52. The second air lock 55 has an inner air lockdoor 56 located proximate to, and in operative communication with, thesecond end 42 of second tunnel chamber 40 and an outer air lock door 57located remote from the inner air lock door 56.

FIG. 2 shows a gap 16 between the sides of container means 70 and sides11 and 12 of apparatus 10 which constitute the sides of reaction chamberor zone 20, the first tunnel chamber or zone 30 and the second tunnelchamber or zone 40. The gap 16 is not drawn to scale. In actualpractice, gap 16 may be quite small where container support means 60 ismerely a floor of apparatus 10 rather than a rail, track or definedpath. The "small" gap minimizes side-to-side movement of container means70 as they are moved respectively through first tunnel chamber 30,reaction chamber 20 and second tunnel chamber 40 or vice versa. Such agap is readily determined without undue experimentation. If containersupport means 60 is a rail, track, channel or other defined path whichrestricts side-to-side movement of container means 70, the gap may belarger if desired.

FIG. 3 is a cross-sectional schematic view of an alternate embodiment70' of a container means 70. In this embodiment, container means 70' hasa solid bottom 74' with no chamber or hollowed-out portion 71 and holdsa bed of solid reactant materials 100. Container means 70' beneficiallymoves along container support means 60 toward a flow of gaseousreactants (not shown). By way of illustration, container means 70',while moving along support means 60 from first air lock 50 (FIG. 1) tosecond air lock 55 (FIG. 1), would face a countercurrent flow ofreactant gases (not shown) introduced via second tunnel chamber conduit45 (FIG. 1) and exiting via first tunnel chamber conduit 35 (FIG. 1). Ifdesired, the flow of reactant gases could be reversed to provide aco-current flow. As used herein, the term "co-current flow" means aparallel flow or a flow which proceeds in the same direction ascontainer means 70 or 70'. In this embodiment, upper reaction chamberconduits 28 are used to introduce reactant gases (not shown) intoapparatus 10. Reaction product gases (not shown) desirably vent throughfirst tunnel chamber conduit 35 (countercurrent flow). In alternateembodiments, reaction product gases could be vented through secondtunnel chamber conduit 45 for co-current flow or simultaneously throughconduits 35 and 45 for a combination of countercurrent and co-currentflow. One or more of conduits 28 (FIG. 1) can be used as sight ports forpyrometer temperature indicators (not shown). In this embodiment, lowerreaction chamber conduits 29 (FIG. 1) may be omitted or closed off.

FIG. 4 schematically depicts, in cross-section, a preferred embodimentof container means 70 in conjunction with a partial sectional view ofapparatus 10. In this embodiment, container means 70 has a hollowed-outchamber 71 configured so as to be in fluid communication with at leastone lower reaction chamber conduit 29. Container means 70 has a porousfloor 72. Floor 72 has an upper or solids receiving surface 73 and alower or gaseous chamber surface 74. The lower surface 74 provides a topor ceiling for chamber 71. The floor 72 has defined therein a pluralityof apertures or passageways 75 which are in fluid communication withboth the chamber 71 and the solids receiving surface 73. In this manner,gaseous reactants (not shown) which enter chamber 71 via conduit 29 willexit chamber 71 via passageways 75 and be distributed throughout a bedof solid reactant materials 100 which is disposed on surface 73. Excessreactant gases and reaction product gases (not shown) may exit Apparatus10 via first tunnel chamber conduit 35, second tunnel chamber conduit 45upper reaction chamber conduits 28 or any combination thereof. Ifdesired, reactant gases or an inert sweep gas or an admixture ofreactant gases and an inert sweep gas may be introduced into apparatus10 via conduit 45, conduit 35 or conduits 28 so long as at least one ofsaid conduits 35, 45 or 28 is used to exhaust or vent reaction productgases (not shown) In addition, one or more of lower reaction chamberconduits 29 may be used to vent excess reactant gases or reactionproduct gases so long as at least one of conduits 28, 29, 35 or 45 isemployed to introduce reactant gases into reaction chamber 20. In otherwords, any combination of conduits may be used to add reactant gases toreaction chamber 20 and exhaust reaction product gases from apparatus 10so long as at least one conduit is dedicated to each function.

A means for moving container means 70 along container support means 60,while not shown, is suitably a hydraulic or pneumatic pushing arm. Othermeans of moving container means 70 from one end of apparatus 10 to theother end and, optionally, back again are readily discernible withoutundue experimentation.

The means for moving container means (not shown) suitably conveyscontainer means 70 or 70' sequentially from first airlock 50 throughfirst tunnel chamber 30, reaction chamber 20, second tunnel chamber 40and into second airlock 55. Each container means 70 beneficiallycontains a bed of solid reactant materials (see. e.g., bed 100 in FIGS.3 and 4). The means for moving container means (not shown) advancescontainer means 70 or 70' through reaction chamber 20 at a ratesufficient to convert substantially all of said solid reactants into adesired reaction product The bed of solid reactant materials 100 isheated to a reaction temperature by heating elements 80 while containermeans 70 or 70' carrying said reactant materials are disposed withinreaction chamber 20. Concurrent with heating, reactant gases (not shown)are conveyed from a source (not shown) via lower reaction chamberconduits 29, chamber 71 of container means 70 located above saidconduits 29 and passageways 75 of said container means 70 through thebed solid reactant materials 100. Reaction product gases (not shown) andexcess reactant gases (also not shown) are exhausted from reactionchamber 20 primarily via first tunnel chamber conduit 35. These gasesmay also be exhausted or vented through one or more of upper reactionchamber conduits 28, second tunnel chamber conduit 45 and lower reactionchamber conduits 29.

If a stream of reactant gases or inert gases (not shown) flowscountercurrent from the second tunnel chamber conduit 45 to the firsttunnel chamber conduit 35, or vice versa, some reaction product gaseswill also be swept from the reaction chamber 20 and out of eitherconduit 35 or conduit 45 as appropriate. As used herein, the term "inertgases" includes noble gases and other gases which do not react withsolid or gaseous reactants under conditions employed to make aparticular product, e.g., aluminum nitride, in apparatus 10.

If container means 70' are used in place of container means 70, reactantgases suitably flow as described in the immediately preceding paragraph.In this case, reactant gases will not enter reaction chamber 20 by wayof conduits 29.

After container means 70 or 70' enter the second airlock 55, a number ofoptions are available. First, container means 70 or 70' may be returnedto first airlock 50 by reversing the direction of travel of thecontainer means. This presumes the presence of a second containersupport means (not shown) as well as a means (also not shown) of movingsaid container means onto said second support means. A second optioninvolves simply removing the reaction product from container means 70 or70' and thereafter reusing said container means. A third option providesan additional passage of solid reactants through apparatus 10 ascontainer means 70 or 70' are removed from second airlock 55 andconveyed by a suitable means (not shown) to first airlock 50. Variationsof these options as well as other options are readily determined bythose skilled in the art without undue experimentation.

Container means 70 and 70' and container support means 60 are suitablyfabricated from graphite. Upper reaction chamber conduits 28 and lowerreaction chamber conduits 29 may also be fabricated from graphite. Thesides, top and bottom of first tunnel chamber 30, reaction chamber 20and second tunnel chamber 40 suitably have a four layer structureconsisting of an inner layer, a first intermediate layer, a secondintermediate layer and an outer layer. The inner layer preferablyconsists of solid pieces of graphite. The first intermediate layerbeneficially contains an insulating material to minimize loss of heat. Avariety of insulation materials, including lamp black, and arrangementsmay be used. The second intermediate layer is desirably made frommasonary materials with firebrick being particularly suitable forreaction chamber 20. The outer layer is desirably formed from steel oranother suitable structural metal. In addition, other structuralmaterials or features, such as additional intermediate layers, may beadded without departing from the scope of the present invention.

Container means used in the following examples either have solid bottoms(70') for countercurrent or co-current configurations (FIG. 3) orperforated bottoms (70) for crosscurrent and combinations ofcrosscurrent, countercurrent, and co-current configurations (FIG. 4).Container means 70 and 70' are suitably 70 to 72 inches (177.8 to 182.9centimeters) long by 9 inches (22.9 centimeters) wide by 3 to 4 inches(7.6 to 10.2 centimeters) tall. The actual dimensions are notparticularly critical so long as container means 70 or 70' arecompatible with mechanical or other means of moving said container meansthrough the combined length of first tunnel chamber 30, reaction chamber20 and second tunnel chamber 40 without undue difficulties such as thoseresulting from excess friction or too much play between the sides ofcontainer means 70 or 70' and the sides of chambers 20, 30 and 40. Thebed of reactant materials 100 suitably has a depth of 0.25 inch (0.64centimeter) to three inches (7.6 centimeters). If desired, a greaterdepth may be obtained with deeper container means 70 or 70'.

The particular form, size and physical properties of such a form ofsolid reactant materials is generally immaterial with a solid bottomcontainer means 70' so long as the solid reactant materials are notswept from the container means by the flow of reactant gases. When usingcontainer means 70, the size or shape of solid reactant materials has anadditional constraint in that it must be sufficient to substantiallypreclude loss of solid reactant materials through passageways 75 andsubsequent interference with movement of the container means 70 alongcontainer support means 60. The solid reactant materials may, forexample, be in the form of a powder or a shape selected from the groupconsisting of pellets, agglomerates, briquettes, tablets, granulates,extrudates or other suitable structure. The solid reactant materials aredesirably in the form of pellets. Pellets may be formed by conventionaltechnology, e.g., by extrusion, tabletting, granulation and the like.

Passageways 75 of container means 70 have two primary size constraints.First, they must provide enough open area to handle the anticipated flowof gaseous reactants, e.g., nitrogen, without so much backpressure thatcontainer means 70 are lifted, even momentarily, from track 60. Second,as noted hereinabove, they must not be so large that solid reactantmaterials easily fall therethrough. By way of illustration only,passageways having a diameter of 0.125 inch (0.32 centimeter) with aspacing of 0.625 inch (1.59 centimeters) center to center are used forthe following examples. Other suitable sizes and spacing are readilydetermined without undue experimentation.

Hollowed-out chamber 71 is beneficially a 0.125 inch (0.32 centimeter)deep pocket cut out of the bottom surface of container means 70. Theactual dimensions of Chamber 71 are not critical so long the chamber islarge enough to handle the anticipated flow of gaseous reactants withoutcausing excessive backpressure.

Heating elements 80 can be heated by flame, electrical elements, or byother heating means. Reaction chamber 20 is suitably heated to atemperature within a range of between about 1500° C. and 1900° C. forpreparation of aluminum nitride Other temperature ranges may be moresuitable for other products. With such a temperature, container means 70and 70' are suitably moved through apparatus 10 at a rate sufficient toprovide adequate residence time, e.g. from about 0.25 to about 6 sixhours. Illustrative rates vary from as low as 0.5 inches per minute toas high as 20 inches per minute. The actual rate will depend uponconstraints such as residence time and size of reaction zone.

The spacing, number and arrangement of lower reaction zone conduits 29are not particularly critical so long they supply sufficient gaseousreactant to the bed of solid reactant materials 100. Each containermeans 70 is suitably located over at least one conduit 29 while saidcontainer means 70 are situated within reaction zone 20. Accordingly,conduits 29 are beneficially spaced evenly within reaction zone 20.Other arrangements are, however, satisfactory.

The spacing, number and arrangement of upper reaction zone conduits 28,like those of lower reaction zone conduits 29, are not particularlycritical The conduits serve one or more of a number of functionsincluding supply of reactant gases, venting of reaction product gasesand providing access for instruments such as pyrometers. The functionsdesired for a particular process dictate actual quantities and spacingof such conduits.

Airlocks 50 and 55, while not shown to scale in FIGS. 1 and 2, aresuitably of a size sufficient to hold, and provide access to, at leastone container means 70 or 70'. Airlocks 50 and 55, particularly thelatter, also serve as a cooling area. As such, the airlocks shouldeither be large enough to serve that function or be connected to anauxiliary cooling or holding area of sufficient size to allow safehandling of said container means and their contents.

Aluminum nitride produced by the method and apparatus of the presentinvention beneficially has a specific surface area of between about 2 m²/gm and about 5 m² /gm. Particle size of the aluminum nitride issuitably less than about 1.5 microns. Oxygen content is desirably about1.5% by weight or less. Iron content within the aluminum nitrideparticles can be higher but is suitably less than about 35 ppm. Thealuminum nitride particles desirably have a silicon content of less thanabout 250 ppm.

Residual carbon in the aluminum nitride product from apparatus 10 can beremoved by exposing the product to air at a temperature of about 700° C.in an apparatus such as a rotary furnace.

The solid reactants which comprise the bed of solid reactant materialsor reaction bed 100 are suitably those which yield the aluminum nitridedescribed hereinabove. The solid reactants beneficially include aluminumoxide and carbon. The carbon is desirably present in an amount which isslightly (one to ten mole percent) in excess of the stoichiometricproportion to aluminum oxide so as to produce carbon monoxide gas duringreaction of aluminum oxide with nitrogen gas. In terms of weightpercent, based upon weight of solid reactant material wherein carbon isa solid reactant, a suitable amount is from about 24 to about 40 weightpercent. The bed of solid reactant materials 100 may also comprise up to1.5 weight percent of calcium oxide, which can act as a catalyst forproduction of aluminum nitride Other sources or derivatives of calcium,as well as other materials which are known to function as catalysts forpreparation of aluminum nitride, may be substituted for calcium oxide.In addition, sintering aids for subsequent densification of aluminumnitride may also be combined with the solid reactant materials. One suchsintering aid is yttria.

The solid reactants, aluminum oxide, carbon and, optionally, calciumoxide are suitably combined and formed into pellets prior to loadinginto container means or trays 70 or 70'. The solid reactants arebeneficially dry-milled for about 4 hours and subsequently mixed withwater and a binder to form an extrudable mixture. Suitable bindersinclude, for example, polyvinyl alcohol, starch, methyl cellulose, andcolloidal alumina, etc. The mixture is extruded into pellets which arebeneficially of a size which facilitates the flow of reactant gasesthrough reaction bed 100, does not substantially impede removal ofgaseous reaction products from reaction bed 100, and minimizes, if noteliminates, plugging of passageways 75 of preferred container means 70.A suitable diameter is 0.25 inch (0.64 centimeter). The pellets arepreferably oven-dried to remove substantially all of their water contentprior to loading into container means 70 or 70'.

Instead of forming pellets, the solid reactants can be granulated toproduce shapes of high porosity. The porous shapes provide sufficientreaction surface area for formation of aluminum nitride.

Reactant gases used in making materials with apparatus 10 are chosen toproduce a desired reaction product. By way of illustration wherein thedesired reaction product is aluminum nitride, reactant gases includenitrogen, mixtures of nitrogen and hydrogen, ammonia, mixtures ofnitrogen or sources of nitrogen plus gaseous sources of carbon Otherpotential sources of reactant gases are known to those skilled in theart

The apparatus 10 is not restricted to production of aluminum nitride. Itshould also be suitable for use in producing other materials viacarbothermal reduction wherein a gaseous reactant is placed in contactwith solid reactants under conditions sufficient to produce the materialOne such reaction product is silicon nitride

By way of illustration only, a suitable combined length of the firsttunnel chamber 30, the reaction chamber 20 and the second tunnel chamber40 is about 60 feet (18.5 meters), with the reaction chamber itselfhaving a length of about eight feet (2.5 meters). First tunnel chamber30, reaction chamber 20 and second tunnel chamber 40 each have a widthof about 10 inches (25.4 centimeters) and a height of about six inches(15.2 centimeters). Trays 70 or 70' typically have dimensions asdescribed hereinabove. Depending upon factors such as rate of movementof trays 70 through the apparatus 10, the amount of solid reactantscontained in said trays, the temperature of reaction zone 20 and therate of flow of gaseous reactants, a production rate from such anapparatus is suitably three pounds per hour or more when the resultantproduct is aluminum nitride

The following examples simply illustrate the present invention and arenot to be construed, by implication or otherwise, as limiting the scopethereof All parts and percentages are by weight and all temperatures arein ° Celsius (° C.) unless otherwise stated.

EXAMPLE 1 AND COMPARATIVE EXAMPLE A

Using a 27 gallon (102.2 liter) mill containing one-half inch (1.3centimeters) high density 99.5% alumina milling media, commerciallyavailable from Coors Ceramics Company, a 25 pound (11.4 kilogram) batchof the following raw materials is dry milled for 4 hours to prepare analuminum nitride (AlN) precursor:

1. 72.0 weight percent alumina powder, commercially available fromAluminum Company of America under the trade designation Alcoa A16-SG;and

2. 28.0 weight percent acetylene carbon black, commercially availablefrom Chevron Chemical Company under the trade designation Shawiniganacetylene black.

The alumina powder has a surface area of 9.46 square meters per gram Ithas the following impurity levels in parts per million: calcium--66;silicon--53; chromium--less than 10; and iron--/80.

A portion of the AlN precursor powder is loaded into one solid bottomtray, hereinafter "Tray A" (see FIG. 3) at a depth of 1/2 inch (1.3centimeter) to provide a total loading of two kilograms (Example 1). Asecond portion is loaded into another solid bottom tray, hereinafter"Tray B", at a depth of 11/2 inches (2.8 centimeters) to provide a totalloading of five kilograms (Comparative Example A). Because the trayshave solid bottoms, there is no upflow of gaseous reactants via lowerreaction zone conduits (see, FIG. 1). The trays are then pushed througha furnace having dimensions as detailed hereinabove (see also FIGS. 1and 2) at a push rate sufficient to give a 96 minute reaction time at amaximum temperature of 1750° C. The reaction time is the estimated totaltime that the reactants are in a portion of the furnace that is at orgreater than the minimum temperature at which the AlN reaction occurs(approximately 1500° C.). A countercurrent nitrogen flow of 1,600 cubicfeet (45.3 cubic meters) per hour is purged through the furnace whilethe trays are contained therein. Exhaust gases are vented through firsttunnel chamber conduit 35 (see, FIG. 1) The trays contain, followingcompletion of passage through the furnace's reaction zone, aluminumnitride. The resultant AlN product in each tray is sampled in sixproduct bed locations (front top and bottom middle top and bottom, backtop and bottom) and analyzed for weight percent oxygen and surface area.The results are summarized in Table 1.

The oxygen content of the AlN product is used as an indication ofreaction completeness. A product oxygen level above 1.5 percent isconsidered an indication, albeit not absolute, of a poorly convertedproduct.

The data presented in Table 1 show that attempts to convert aluminapowder to aluminum nitride powder in a thick bed (Tray B) result in muchgreater variability of product oxygen content as well as a larger rangeof resultant oxygen contents than similar attempts with a comparativelythin bed (Tray A). This is believed to be due to heat transfer and gastransport limitations inherent in thicker powder beds. All oxygencontents in Tray A save one are less than 1.5 percent.

                  TABLE 1                                                         ______________________________________                                                               Weight   Surface                                                Sample        Percent  Area                                          Tray     Location      Oxygen   m.sup.2 /g                                    ______________________________________                                        A        Front Top     1.16     2.81                                          A        Front Bottom  1.20     2.92                                          A        Middle Top    1.39     2.93                                          A        Middle Bottom 1.27     3.10                                          A        Back Top      1.76     3.26                                          A        Back Bottom   1.36     2.93                                          B        Front Top     14.38    4.63                                          B        Front Bottom  8.30     2.79                                          B        Middle Top    2.78     3.52                                          B        Middle Bottom 7.87     3.23                                          B        Back Top      4.07     3.47                                          B        Back Bottom   8.89     2.75                                          ______________________________________                                    

While this may indicate that a portion of the resultant powder may havean excessively high oxygen content, the average oxygen content ofaluminum nitride powder contained in that tray is clearly acceptable.Conversely, all oxygen contents in Tray B are clearly above, oftenconsiderably above, 1.5 percent. As such, the average oxygen content ofpowder contained in Tray B is also above such a desirable oxygen level.

EXAMPLE 2

Using the procedure and apparatus of Example 1, a 25 pound (11.4kilogram) batch of aluminum nitride precursor powder is prepared from71.7 percent of the same alumina as in Example 1, 28.0 percent of thesame acetylene carbon black as in Example 1 and 0.3 percent calciumoxide.

The aluminum nitride precursor powder is mixed in a ribbon blender with30% water and 5% polyvinyl alcohol. The resultant mixture is extrudedinto 1/4 inch (0.6 centimeter) extrudates or pellets. The pellets areoven dried to a water content of less than 2.0%.

Solid bottom trays (see, FIG. 3) are loaded to a depth of 0.25 inch (0.6centimeter) with the pellets to provide a total solid reactant charge oftwo kilograms. The density difference between pellets and powderaccounts for a shallower depth of pellets to attain a particularloading. The trays are then pushed through the same furnace as inExample 1 at a rate sufficient to provide a 48 minute reaction time at1750° C. maximum temperature. A countercurrent N₂ flow of 1500 CFH (42.5cubic meters per hour) is used. The average oxygen content of theresulting AlN product is 1.26%.

COMPARATIVE EXAMPLE B

The process and composition of Example 2 is duplicated save forincreasing the depth of pellets in the trays fourfold to one inch (2.54centimeters).

The resulting AlN product has an average oxygen content of 20.6%. Thisindicates an unacceptably low conversion of alumina to AlN. The poorconversion is believed to result from a combination of mass transfer andheat transfer limitations.

The data presented in Example 2 and Comparative Example B show thatpellets, like powder, provide a much more acceptable product in shallowbed than in a comparatively thick bed. As such, there is no significantdifference in terms of oxygen content in using pelletized, rather thanpowdered, solid reactants in a solid bottom tray or container means.

EXAMPLE 3

Using a modified furnace wherein lower reaction zone conduits 29 (see.FIG. 1) are used to supply gaseous nitrogen to the hollowed-out chamber71 of container means 70 (see, FIG. 4), the procedure of Example 2 isduplicated. Exhaust gases are vented through first tunnel chamberconduit 35 as in Example 1. The hollowed-out chamber is a 1/8 inch (0.3centimeter) deep cavity routed out of the bottom of each tray. Thecavity acts as a nitrogen distributor which evenly supplies nitrogen to1/8 inch (0.3 centimeter) diameter holes which are spaced 5/8 inches(1.6 centimeter) from center to center across the entire floor of thetray. The conduits 29 are actually graphite tubes, each of which has anexit which is flush with the floor 60 of reaction zone 20 of thefurnace. The depth of pellets in the trays is one inch (2.5 centimeters)as in Comparative Example B. Gaseous nitrogen flow through conduits 29is at a rate of 1600 CFH (45.3 cubic meters per hour).

The average oxygen content of the resulting AlN product is 1.22%. Theproduct has a surface area of 3.03 m² /gm. These results show a dramaticimprovement in conversion over that obtained in Comparative Example B.

EXAMPLE 4 AND COMPARATIVE EXAMPLE C

AlN precursor pellets prepared as in Example 2 are loaded into a solidbottom tray (Comparative Example C) and a perforated bottom tray(Example 4) to a depth of 0.75 inch (1.9 centimeters). The solid bottomtray and the perforated bottom tray are identical, respectively, tothose used in Examples 1 and 3. Each of the two trays is pushed throughthe apparatus at a rate sufficient to provide a reaction time of about96 minutes with a maximum temperature of 1750° C. In the case of theperforated tray gaseous nitrogen is supplied to the hollowed-outchamber, as in Example 3, at a rate of 1500 CFH (42.5 cubic meters perhour). The AlN product in each tray is sampled in 6 locations, as inExample 1, and analyzed for wt. % oxygen and surface area as determinedfrom Brunauer, Emmet, Teller (B.E.T.) surface area analysis. The resultsare summarized in Tables 2 and 3.

                  TABLE 2                                                         ______________________________________                                        Solid Tray                                                                                     Percent  Surface                                             Sample Location  Oxygen   Area m.sup.2 /gm                                    ______________________________________                                        Front Top        4.32     3.28                                                Front Bottom     11.08    2.99                                                Middle Top       1.25     3.11                                                Middle Bottom    1.44     2.88                                                Back Top         3.55     3.03                                                Back Bottom      7.93     2.98                                                ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Perforated Tray                                                                                Weight                                                                        percent  Surface                                             Sample Location  Oxygen   Area m.sup.2 /gm                                    ______________________________________                                        Front Top        1.24     2.99                                                Front Bottom     1.22     2.91                                                Middle Top       1.10     2.63                                                Middle Bottom    1.03     2.81                                                Back Top         1.04     2.70                                                Back Bottom      1.13     2.87                                                ______________________________________                                    

A comparison of the data presented in Tables 2 and 3 shows that theperforated tray (Example 4) produces a more uniform product quality thanthe solid tray (Comparative Example C). Because of the similarity of allfactors other than the pattern of flow of gaseous nitrogen, the improvedresults are believed to follow from a more uniform distribution of thereaction gases. The reaction in the perforated tray also proceeds at afaster rate as evidenced by the lower oxygen values. The product sampledfrom the bottom layer on the solid tray shows early indications ofproduct coarsening even though the precursor is not fully converted.This may be caused by an insufficient N₂ supply near the bottom of thebed with the solid tray.

Although preferred embodiments have been specifically described andillustrated herein, it will be appreciated that many modifications andvariations of the present invention are possible, in light of the aboveteachings, within the purview of the following claims, without departingfrom the spirit and scope of the invention.

What is claimed is:
 1. A method for continuously producing aluminumnitride via carbothermal reduction of aluminum oxide, the methodcomprising:(a) providing at least one discrete aliquot of solid reactantmaterial, each aliquot being disposed within a separate container, eachcontainer having defined therein a means for receiving gaseous reactantsand distributing said gaseous reactants in a generally uniform mannerthroughout said aliquot, said solid reactant materials comprisingaluminum oxide and, optionally, carbon, said gaseous reactantscomprising nitrogen and, optionally, a source of carbon; (b) passing thecontainer(s) at least once through a heated reaction zone at a rate andfor a period of time sufficient to heat said aliquot to a temperaturesufficient to initiate a reaction between the gaseous reactants andsolid reactant materials to produce aluminum nitride; (c) supplying thegaseous reactants to said container at a rate sufficient to convert thesolid reactant materials to aluminum nitride; (d) removing gaseousreaction products from the reaction zone at a rate sufficient to formaluminum nitride with a controlled particle size and substantiallypreclude formation of aluminum oxynitride or aluminum oxycarbide.
 2. Themethod of claim 1 wherein the period of time is between about 0.25 hoursand about 6 hours and the temperature is between about 1500° C. andabout 1900° C.
 3. The method of claim 1 wherein the solid reactantmaterials are in the form of a powder or a shape selected from the groupconsisting of briquettes, tablets, granulates, agglomerates, extrudatesor of pellets.
 4. The method of claim 1 wherein the solid reactantmaterials comprise aluminum oxide, carbon and calcium oxide, the calciumoxide being present in an amount of from about 0.05 to about 1.5 percentby weight, based upon weight of solid reactant materials, the carbonbeing present in an amount which is slightly in excess of itsstoichiometric proportion to aluminum oxide, and the aluminum oxidecomprising the balance.
 5. The method of claim 4 wherein the amount ofcarbon is from about one to about ten mole percent greater than thestoichiometric proportion of carbon to aluminum oxide.
 6. The method ofclaim 4 wherein the amount of carbon is from about 24 to about 40percent by weight, based upon weight of solid reactant materials.
 7. Themethod of claim 1 wherein the gaseous reactants are heated prior tobeing supplied to the container.
 8. The method of claim 1 wherein thegaseous reactants are supplied at a rate sufficient to convertsubstantially all of the solid reactant materials to aluminum nitride.9. The method of claim 1 wherein an inert sweep gas aids in removal ofgaseous reaction products.
 10. The method of claim 1 wherein the gaseousreactants flow countercurrent to movement of the containers.
 11. Themethod of claim 1 wherein the gaseous reactants flow co-current tomovement of the containers.